1. Field of the Invention
This invention relates generally to electrical signal amplifiers, and more particularly to a differential amplifier having a balanced output and common-mode feedback.
2. Description of Related Art
A balanced or fully differential amplifier typically requires common-mode feedback to ensure that the amplifier operates in the region where its full differential dynamic range can be utilized. Certain implementations of common-mode feedback, however, may add additional voltage offset to the amplifier, move parasitic poles closer to the dominant pole, degrade stability or increasing the power required to obtain desired stability, causes common-mode to differential-mode signal conversion due to random mismatch of components, and/or may result in having latching or dead-zones from which the amplifier cannot recover and operate normally.
In an attempt to overcome these drawbacks, common-mode feedback using input stage tail current servoing and/or common-mode servoing using input stage active loads or folded-cascode active loads have been employed.
While common-mode feedback using input stage tail current servoing can have dead zones or start-up problems, it does nevertheless have certain desirable properties. For example, it does not permit common-mode to differential conversion and tends to exhibit a relatively enhanced bandwidth and offset properties. Common-mode servoing using input stage active loads or folded-cascode active loads can increase differential offset due to component mismatch, can degrade stability, can increase supply current required to maintain stability, and can allow common-mode to differential conversion.